In a fiber laser the laser-active medium is incorporated in a light waveguide. Laser activity of the fiber is attained in particular by doping the fiber core with ions of rare earths. In numerous laser transitions of rare earth ions it was possible for the first time to observe laser emission in fiber lasers, in particular since it has been possible to use, besides silicate glasses, fluorite glasses, above all fluor zirconate glass ZBLAN, as a host material. In contrast to silicate glasses fluorite glasses have smaller effective photon energies, which results in smaller rates of non-radiative decay, longer durations of excited states and larger amplification cross-sections. Fluorite glasses allow high quantum efficiencies and an efficient excitation into elevated energy levels to be attained by absorption out of excited states such that laser emission out of states, in which the excitation energy is larger than the quantum energy of the exciting light (upward conversion laser), is possible.
Active fibers can generally be produced by doping the fiber core with laser-active materials. Here, excitation of the ions is effected via a pump light source for generating pump light to be coupled into the fiber. The pump light is longitudinally irradiated into the fiber such that said light is absorbed by the ions. The pump light is focussed, with the aid of a lens, onto the front side of the fiber, coupled into the fiber core and guided there.
If only a portion of the light coupled into the fiber is absorbed by the corresponding ions, a high-performance pump light source is required for generating an adequate laser intensity. For increasing the efficiency of the laser, i.e. in particular of the amount of absorbed light from the pump light source, it is common practice to feed back into the fiber, with the aid of a mirror, light emerging at the exit end of the fiber, which has not yet been absorbed by the ions contained in the fiber. Further, it is common practice to evaporate the resonator mirror directly onto the fiber end.
Such a fiber laser is e.g. known from DE 196 36 236 A1. The diode laser-pumped multimode waveguide laser comprises a diode laser. With the aid of a collimating optics the light emitted by the diode laser is coupled into the fiber at the entrance end of the fiber. A mirror is evaporated onto the entrance end of the fiber. The mirror reflects the pumping wavelength generated by the diode laser only to a very small extent. However, the mirror arranged at the entrance end reflects the generated laser light extremely well. Thus, at the entrance end of the fiber almost only light with the pumping wavelength can emerge from the fiber. The opposite fiber end, the exit end of the fiber, can be antireflection-coated for the laser wavelength to be generated.
For feeding laser light generated in the fiber back into the fiber, a mirror is arranged in a spaced relationsship to the exit end of the fiber. The light reflected by this resonator mirror is focussed with the aid of a lens disposed between the exit end of the fiber and the resonator mirror and fed back into the fiber.